Copyright © 2009 Pearson Education, Inc.
Including some materialsfrom lectures by
Gregory AhearnUniversity of North Florida
Ammended byJohn Crocker
Chapter 6
Capturing Solar Energy:
Photosynthesis
Copyright © 2009 Pearson Education Inc.
6.1 What Is Photosynthesis?
Life on earth depends on photosynthesis.• Photosynthesis is the capturing and
conversion of sunlight into chemical energy.• Virtually all life depends directly or indirectly
on the energy captured by plants and stored as sugars.
• Before photosynthesis, there was little oxygen on Earth, and therefore, no organisms that used oxygen.• All present-day organisms that use oxygen
as their respiratory gas depend upon photosynthesis to generate new oxygen.
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6.1 What Is Photosynthesis?
Photosynthesis converts carbon dioxide and water to glucose.• The chemical reaction for photosynthesis:
• 6 CO2 + 6 H2 0 + light energy C6 H12 O6 + 6 O2
• Plants, seaweeds, and single-celled organisms all show the basic aspects of photosynthesis.
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6.1 What Is Photosynthesis?
Plant photosynthesis takes place in leaves.• Leaves are the main location of
photosynthesis.• Plants have thin leaves so sunlight can
penetrate.• Plant leaves have a large surface area to
expose them to the sun.• Plant leaves have pores to admit CO2 and
expel O2 , called stomata (singular, stoma).
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6.1 What Is Photosynthesis?
Leaf cells contain chloroplasts.• Photosynthesis occurs in chloroplasts, in layers
of cells called the mesophyll.• Chloroplasts contain a semifluid medium called
stroma, which contains sacs called thylakoids within which photosynthesis occurs.
Copyright © 2009 Pearson Education Inc.Fig. 6-1
6.1 What Is Photosynthesis?
An overview of photosynthetic structures
mesophyll cells
chloroplastsvein stoma
outer membraneinner membranethylakoidstroma
Internal leaf structure
Chloroplast in mesophyll cell
Leaves
(b)
(c)
(a)
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6.1 What Is Photosynthesis?
Photosynthesis consists of light-dependent and light-independent reactions.• These reactions occur at different locations in
the chloroplast.• The two types of reactions are linked by the
energy-carrier molecules adenosine triphosphatase (ATP) and nicotinamide adenine dinucleotide phosphate (NADPH).
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6.1 What Is Photosynthesis?
Light-dependent reactions• Occur in the membranes of the thylakoids• Light is captured here and stored in ATP and
NADPH.• H2 O is consumed and O2 is given off.
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6.1 What Is Photosynthesis?
Light-independent reactions• Enzymes in the stroma use ATP and NADPH
produced by light-dependent reactions to make glucose and other molecules.
• CO2 is consumed in the process.• ATP and NADPH are converted to low-energy
ADP and NADP+.• These low-energy molecules are re-charged
to ATP and NADPH when recycled in light- dependent reactions.
Copyright © 2009 Pearson Education Inc.Fig. 6-2
6.1 What Is Photosynthesis?
An overview of photosynthesis: light- dependent and light-independent reactions
LIGHT-DEPENDENT REACTIONS (thylakoids)
LIGHT-INDEPENDENT REACTIONS
(stroma)
depleted carriers
(ADP, NADP+)
H2 O O2
CO2 glucose
energized carriers
(ATP, NADPH)
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The Energy in Visible Light
The sun radiates electromagnetic energy
Visible light is radiation falling between 400- 750 nanometers of wavelength
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Light Captured by Pigments
Packets of energy called photons have different energy levels depending on wavelength• Short-wavelength photons (indigo, UV, x-rays)
are very energetic• Longer-wavelength photons have lower
energies (red, infrared)
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Light Captured by Pigments
Action of light-capturing pigments• Absorption of certain wavelengths
(light is “trapped”)• Reflection of certain wavelengths
(light bounces back)• Transmission of certain wavelengths
(light passes through)
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Light Captured by Pigments
Absorbed light drives biological processes when it is converted to chemical energy
Common pigments found in chloroplasts include:• Chlorophyll a and b• Accessory pigments such as
carotenoids
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Light Captured by Pigments
Pigment absorbs visible light
Chlorophyll a and b absorb violet, blue, and red light but reflect green light (hence they appear green)
Carotenoids absorb blue and green light but reflect yellow, orange, or red (hence they appear yellow-orange)
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Copyright © 2009 Pearson Education Inc.
Why Autumn Leaves Turn Color
Both chlorophylls and carotenoids are present in leaves• Chlorophyll breaks down before carotenoids in
dying autumn leaves revealing yellow colors• Red (anthocyanin) pigments are synthesized
by some autumn leaves, producing red colors
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6.2 How Is Light Energy Converted To Chemical Energy?
Light is first captured by pigments in chloroplasts.• Membranes of choroplast thylakoids contain
several types of pigments (light-absorbing molecules).• Chlorophyll • Other accessory pigments including
carotenoids
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6.2 How Is Light Energy Converted To Chemical Energy?
Captured sunlight energy is stored as chemical energy in two carrier molecules• Adenosine triphosphate (ATP)• Nicotinamide adenine dinucleotide
phosphate (NADPH)
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6.2 How Is Light Energy Converted To Chemical Energy?
The light-dependent reactions generate energy-carrier molecules.• Light-dependent reactions take place in
photosystems found in the thylakoid membranes.
• Each photosystem consists of an assemblage of proteins, chlorophyll, accessory pigment molecules, and electron-carrier molecules.
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6.2 How Is Light Energy Converted To Chemical Energy?
In thylakoids, there are thousands of photosystems of two types.• Photosystem I• Photosystem II• Each Photosystem is associated with a chain
of electron carriers
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6.2 How Is Light Energy Converted To Chemical Energy?
Each photosystem consists of two major subsystems.• A light-harvesting complex collects light
energy and passes it on to a specific chlorophyll molecule called the reaction center.
• An electron transport system (ETS) transports energized electrons from one molecule to another.
Copyright © 2009 Pearson Education Inc.Fig. 6-4
6.2 How Is Light Energy Converted To Chemical Energy?
Structures associated with the light- dependent reactions
thylakoids
PS II
chloroplastwithin thylakoid membrane
PS IETC ETCreaction centers
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6.2 How Is Light Energy Converted To Chemical Energy?
Photosystem II generates ATP.• Step 1: The light-harvesting complex passes
light to the reaction center.• Step 2: Electrons of the reaction center
become energized.• Step 3: The energized electrons jump to the
ETS and jump from molecule to molecule, releasing energy at each step.
• Step 4: The released energy powers reactions that synthesize ATP.
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6.2 How Is Light Energy Converted To Chemical Energy?
Photosystem I generates NADPH.• Step 5: The light-harvesting complex passes
light to the reaction center.• Step 6: Activated electrons from the reaction
center are passed to the ETS and are replaced by electrons coming from the ETS of photosystem II.
• Step 7: Electrons jump from one molecule of the ETS to another, until they reach NADP+.
• Step 8: Each molecule of NADP+ picks up two electrons, forming NADPH.
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6.2 How Is Light Energy Converted To Chemical Energy?
• Step 9: The breakdown of H2 O provides the replacement electrons to keep the process continuing, through the reaction:
H2 O
½ O2 + 2H+ + 2e–
• The two electrons are donated to photosystem II.• The hydrogen ions are used to convert NADP+ to
NADPH.• Oxygen atoms combine to form a molecule of
oxygen gas (O2 ), which is given off to the atmosphere.
Copyright © 2009 Pearson Education Inc.
reaction center
photosystem II
photosystem I
synthesisenergy to drive
sunlight
9
5
8
7
36
1
4
2
NADPH
2 H+H2 O
e–
e–
H+NADP+
ATP
e–
e–
+
1/2 O2
ener
gy le
vel o
f ele
ctro
nsw
ithin thylakoidm
embrane
electron transport chain
Fig. 6-5
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Copyright © 2009 Pearson Education Inc.
6.2 How Is Light Energy Converted To Chemical Energy?
Splitting water maintains the flow of electrons through the photosystems.• Electrons from the reaction center of
photosystem II flow through the ETS of photosystem II to the reaction center of photosystem I, forming NADPH.
• Photosystem II’s reaction center must be supplied with new electrons to keep the process continuing.
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Maintaining Electron Flow Redux
Electrons leaving PS II replaced when H2 O is split:
• H2 O ½O2 + 2H+ + 2e-
• Two electrons from water replace those lost when 2 photons boost 2 electrons out of PSII
• Two hydrogen ions used to form NADPH• Oxygen atoms combine to form O2
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6.3 How Is Chemical Energy Stored in Glucose Molecules?
The ATP and NADPH generated in light- dependent reactions are used in light- independent reactions to make molecules for long-term storage.• These reactions occur in the fluid stroma that
surrounds the thalakoids, and do not require light.
• In the stroma, ATP and NADPH are used with CO2 and H2 O to synthesize the storage form of energy—glucose.
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6.3 How Is Chemical Energy Stored in Glucose Molecules?
The C3 cycle captures carbon dioxide.• Step 1: CO2 from air combines with a five-carbon
sugar, ribulose biphosphate (RuBP), and H2 O to form phosphoglyceric acid (PGA).
• Step 2: PGA receives energy input from ATP and NADPH to form glyceraldehyde-3-phosphate (G3 P).
• Step 3: Two G3 P molecules (three carbons each) combine to form one molecule of glucose (six carbons).
• Step 4: 10 G3 P molecules powered by ATP are used to regenerate six molecules of RuBP to restart the cycle.
Copyright © 2009 Pearson Education Inc.Fig. 6-6
6.3 How Is Chemical Energy Stored in Glucose Molecules?
The C3 cycle of carbon fixation
12
12
12
12G3P12
12PGA G3P
synthesis uses energy
RuBP
6
6
6 CO2
6
RuBP synthesis uses energy and 10 G3Ps
glucose
Carbon fixation combines CO2 with RuBP
2 G3Ps available for synthesis of glucose
C3 cycle
ADP
ATP
ATP
NADPH
NADP+
ADP
1
4
3
2
C
C C C C C C C C
C C C
C C C C C C
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6.4 What Is The Relationship Between Light-Dependent And Light-Independent Reactions?
Photosynthesis includes two separate sets of reactions (light-dependent and light- independent) that are closely linked.
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6.4 What Is The Relationship Between Light-Dependent And Light-Independent Reactions?
Light-dependent reactions capture solar energy; light-independent reactions use captured energy to make glucose.• Energy-carrier molecules provide the link
between these two sets of reactions.• Light-dependent reactions of thylakoids use
light to charge ADP and NADP+ to make ATP and NADPH.
• ATP and NADPH move to the stroma where they provide energy to synthesize glucose.
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O2
H2 O
ATP
ADP
glucose
NADP+
CO2
Light-dependent reactions occur in thylakoids
energy from sunlight
Light- independent reactions (C3 cycle) occur in stroma
chloroplast
NADPH
Fig. 6-7
6.4 What Is The Relationship Between Light-Dependent And Light-Independent Reactions?
Two sets of reactions are connected in photosynthesis.
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6.5 How Does the Need To Conserve Water Affect Photosynthesis?
Photosynthesis requires carbon dioxide; porous leaves would allow the entry of CO2 , but would also result in the loss of H2 O.
Evolution of the stomata resulted in pores that could open, letting in CO2 , but also to close, to restrict H2 O losses.
Closing stomata to prevent H2 O loss also restricts the release of O2 , produced by photosynthesis, to the atmosphere.
Copyright © 2009 Pearson Education Inc.Fig. 6-8
6.5 How Does the Need To Conserve Water Affect Photosynthesis?
Stomata
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6.5 How Does the Need To Conserve Water Affect Photosynthesis?
When stomata are closed to conserve water, wasteful photorespiration occurs.• In hot, dry conditions, plant stomata are
closed much of the time, reducing internal CO2 concentrations and increasing O2 concentrations.
• Increased O2 reacts with RuBP (instead of CO2 ) in a process called photorespiration.
• Photorespiration does not produce useful cellular energy, and prevents the C3 synthesis of glucose.
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6.5 How Does the Need To Conserve Water Affect Photosynthesis?
Alternative pathways reduce photorespiration.• Some plants have evolved metabolic
pathways that reduce photorespiration.• These plants can produce glucose even under
hot and dry conditions.• The two most important alternative pathways
are:• The C4 pathway• Crassulacean acid metabolism (CAM)
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6.5 How Does the Need To Conserve Water Affect Photosynthesis?
Plants capture carbon and synthesize glucose in different places.• Typical plants (C3 plants) fix carbon and
synthesize glucose as a result of the C3 cycle in mesophyll cells.
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bundle- sheath cell
mesophyll cell
C3 plant
In a C3 plant, carbon capture and glucose synthesis are in mesophyll cells
(a)
Fig. 6-9a
6.5 How Does the Need To Conserve Water Affect Photosynthesis?
C3 plant
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6.5 How Does the Need To Conserve Water Affect Photosynthesis
The C4 pathway includes two stages that take place in different parts of the leaf.• In the first stage, CO2 is captured in mesophyll
cells in the presence of high O2 , producing a four-carbon molecule.
• The four-carbon molecule is transferred from mesophyll cells to the bundle-sheath cells where the four-carbon molecule is broken down to CO2 .
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bundle- sheath cell
mesophyll cell
C4 plant
In a C4 plant, carbon capture is in mesophyll cells, but glucose is synthesized in bundle-sheath cells
(b)
Fig. 6-9b
6.5 How Does the Need to Conserve Water Affect Photosynthesis
C4 plant
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6.5 How Does the Need to Conserve Water Affect Photosynthesis
C4 plants capture carbon and synthesize glucose in different places.• In the sheath-bundle cells, the released CO2
proceeds to the second stage of the pathway— the regular C3 cycle—without excess O2 interfering with the process.
• Many C4 plant species are grasses, and are agriculturally important species such as sugar cane, corn, and sorghum.
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6.5 How Does the Need to Conserve Water Affect Photosynthesis
CAM plants capture carbon and synthesize glucose at different times.• In CAM plants, photorespiration is reduced by
fixing carbon in two stages that take place in the same cells but at different times of the day.
• At night, with open stoma, reactions in mesophyll cells incorporate CO2 into the organic acid molecules that are stored in vacuoles.
• During the day, with stoma closed, the organic acids release their CO2 and the regular C3cycle proceeds.
Copyright © 2009 Pearson Education Inc.Fig. 6-10
6.5 How Does the Need to Conserve Water Affect Photosynthesis
Two ways to reduce photorespiration—in different places and times
Steps in separate places Steps at separate times
CO2 is incorporated into four-carbon molecules
Four-carbon moleculesrelease CO2 to the C3 cycleC3
cycle
CO2 CO2
CO2 CO2
C4 CAM
mesophyll cell
bundle-sheath cell
night
daymesophyll cell
C3 cycle
(a) (b)
1
2
C C C C C C C C